1. Field of the invention
The present invention relates to an improved process and catalyst system for the isomerization of halothiophenes. More particularly, the invention pertains to an improved process for the isomerization of 2-halothiopehenes to 3-halothiophenes by reacting a 2-halothiophene with a zeolite catalyst and a base.
2. Description of the related art
Halogenated thiophenes are useful intermediates for the preparation of pharmaceuticals, plant protection agents, photovoltaic and conductive polymers, liquid crystal polymers and organic light-emitting diodes. As discussed in U.S. Pat. No. 4,604,470, which is incorporated herein by reference, 3-halothiophenes are particularly useful, yet typically are more difficult and expensive to produce compared to their 2-halothiophene counterparts. For example, while 2-chlorothiophene can be synthesized by chlorination of thiophene, 3-chlorothiophene is not accessible via this route. Other synthesis routes are therefore required for its preparation, for example, the reaction of 3-bromothiophene with CuCl. Further, 2-bromothiophene can be obtained, for example, by the bromination of thiophene in acetic acid, while 3-bromothiophene is prepared by the dehalogenation of 2,3,5-tribromothiophene with zinc dust in acetic acid.
One typical method used for the synthesis of 3-halothiophenes is the isomerization of its corresponding 2-halothiophene using a zeolite catalyst. However, such processes result in an inadequate product yield due to the decomposition of the thiophene ring during the isomerization process. In fact, nearly all of the methods currently known for the preparation of 3-chloro-, 3-bromo or 3-iodothiophene either require very expensive starting substances or give inadequate yields. Accordingly, previous known isomerization processes have suffered from low product yields, reduced catalyst lifetime, and hence, increased cost. Consequently, there is a need in the art for simpler and less expensive methods for the formation of 3-halothiophenes, particularly on an industrial scale.
It has now been found that 2-halothiophenes can be isomerized to 3-halothiophenes in good yields by the reaction of a 2-halothiophene in the presence of a zeolite catalyst and a base. It has been unexpectedly discovered that by the addition of a base as an additive to the acidic zeolite catalyst, not only will the desired isomerization reaction still take place, but the base additive suppresses the side-reaction that results in the decomposition of thiophene rings. This also has been found to increase the lifetime of the catalyst. Accordingly, the process allows for a reduction in the amount of catalyst needed for the synthesis reaction. It has been found the process of the invention is capable of producing a yield concentration of over 80% 3-halothiophene isomer without any major decomposition of the thiophene ring from the 2-halothiophene isomer reactant, and at a low cost.